The present invention generally relates to injection molding, and in particular, to a technique in which a gas is injected into the molten resin in a cavity which forms the molded article. With such techniques, a high pressure gas has been needed in the past to inject an inert gas into the molten resin. The high pressure gas has typically been provided by connecting a hydraulic cylinder directly to a gas compressor piston. The maximum thrust, F.sub.1 MAX, necessary for compression is best represented by the formula: EQU F.sub.1 MAX=A.sub.1 P.sub.1 MAX
In the above formula, A.sub.1 is the compression cylinder's sectional area, and P.sub.1 MAX is the final compression pressure.
As might be expected, such a method requires a great amount of thrust, and thus, usually employs a large oil pressure generating unit. Furthermore, the gas temperature is abruptly increased because maximum pressure is obtained through one-stage compression. Also, the stress applied to members forming the compression cylinder must be within allowable stress limits of the material, which is a function of the gas temperatures which result from compression. Thus, the members must be extremely large in thickness. The manufacturing costs, therefore, are often unavoidably high, and suitable locations for installation of compression cylinders is limited.
While two-stage and three-stage gas compressors, with intermediate gas coolers have been developed in the art, they are normally intricate and very complex in design, and the manufacturing costs are also high.
The pressure of the gas injected into the molding is, in general, controlled by setting the hydraulic pressure applied to the hydraulic cylinder. Unfortunately, optimal control of this parameter has been difficult to obtain. For example, in a method disclosed in Japanese Patent Application (OPI) No. 139716/1988 (the term "OPI" as used herein means an "unexamined published application"), a second compression is carried out by using another high pressure inert gas. However, in that method no pressure control is utilized, and there is only one-way compression. Thus, the pressure is not economically used.